The present invention relates to a process to control the hydraulic pressure provided by a hydraulic pump and a hydraulic pump with pressure control.
Processes and devices for controlling the hydraulic pressure provided by a hydraulic pump are known in the art. Typically, modern hydraulic pumps, particularly for power assisted steering applications, are assembled from modular subassemblies or pump components that are arranged relative to each other. The pumps are designed to produce a hydraulic pressure matching the system requirements and, in the line coming from the pump, or in a pump area subject to operating pressure, a pressure control valve is arranged. The design of this pressure control valve can vary. Typical are springloaded pressure control valves, which open in case of overpressure and may also carry oil in a bypass leading back to the reservoir.
Despite increasing miniaturization and a production and materials engineering related decrease in the costs of the corresponding components, particularly pressure control valves, said components continue to be a major cost factor in the production of hydraulic systems using hydraulic pumps. They are separate units that must be separately produced and separately installed. This substantially increases production costs, installation costs and overall system production costs.
On the other hand, such pressure control valves cannot currently be dispensed with since an undesirable overpressure in the hydraulic system may damage other components, which for cost reasons are all designed for minimum pressure ratios, or may produce substantial malfunctions.
Based on the described state of the art, the object of the present invention is to define a process and a hydraulic pump permitting pressure control on the one hand and elimination of separate and/or additional pressure control valves on the other hand.
To attain this object on the process side, the invention proposes a process to control the hydraulic pressure provided by a hydraulic pump that has at least two pump components that are connected in pressure tight manner by fastening means in order to form a hydraulically pressurized volume, characterized in that at least one of the pump components is arranged on another such that the former can be moved relative to the other pump component above a given hydraulic pressure within the pump interior to open the pressurized volume at least slightly.
The process according to the invention has the significant advantage of component reduction, which is expressed, in particular, by the elimination of conventional pressure control valves. The manufacturing method, however, hardly differs from the previous manufacturing method so that pure production cost savings result.
An exemplary design for a power assisted steering application of a hydraulic pump, which forms part of prior art, comprises a valve assembly, a pump body, a resonator chamber and a resonator cover. Together, these components are disposed within a housing providing an oil reservoir from which practically only the valve assembly protrudes for connecting the hydraulic intake and return lines.
According to the invention, the valve assembly, pump body, resonator and resonator cover are now fastened by means of screws for which force F is applied. The resonator cover is typically pressurized with the operating pressure. Starting from a certain pressure, the hydraulic force acting on the resonator cover becomes greater than the prestressing force of the bolts, which is adjusted to this pressure, so that gaps may form between valve assembly, pump body, resonator and resonator cover. A small volumetric oil flow passes through these gaps into the surrounding reservoir area, which creates the desired pressure control.
This object is attained in that the screws or bolts, the mechanical properties of which are all known and predefined, are tightened with a given torque. The elastic properties of the screw material together with the given torque makes it possible positively to predetermine the liftoff pressure or the pressure at which gaps are formed.
Until now, torques were selected to be high enough so that no gaps could form in the pressure areas where the pressure control valve had not already previously responded. For this reason static seals are inserted between the individual pump components. This practically prevents a volumetric oil flow. The pressure is controlled by means of conventional pressure control valves.
In contrast, the process according to the invention represents a significant improvement since, through the type of mounting provided, the conventionally assembled pump now becomes a self-contained pressure control system in which the holding forces are placed in direct relation to the internal pressure produced.
Independent of the pure fastening force, the type of the fastening elements can also exercise the same process function. It is quite feasible, for example, to assemble the individual pump components by means of clamp-type mechanical fastening elements, which for their part have sufficient elastic properties to exercise a pressure control function in accordance with the process in that they permit the formation of gaps starting from a specific hydraulic pressure within the interior of the pump components. Besides, gaps may form between two or also between several pump components.
The invention provides a completely novel hydraulic pump, which itself has means to control the hydraulic pressure provided, namely fastening means that permit relative motion of pump components above a given hydraulic pressure within the pump interior to open the pressurized volume at least slightly.
Advantageously, these means are the fastening bolts for the resonator cover, which are tightened with a given torque, whereby the mechanical properties of the bolts are given and defined.